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“For the third time since the OPERA detector began receiving beam in 2006, the experiment has caught a muon neutrino oscillating into a tau neutrino.

For the third time ever, scientists have seen the particle transformation that explains the mystery of the ‘missing neutrinos’—particles we expect to rain down from the Sun and Earth’s atmosphere at higher rates than observed.

Neutrinos are light particles that come in three types, or flavors, each associated with a different subatomic particle: an electron, a muon or a tau. One of the biggest surprises that came with the discovery of neutrinos was that they could change from flavor to flavor.

Members of the OPERA experiment announced today the observation of a muon neutrino that had switched flavors to a tau neutrino. OPERA scientists, based at Gran Sasso National Laboratory in Italy, have caught this rare event only twice before, once in 2010 and once in 2012.

The new observation ‘is an important confirmation of the two previous observations,’ says Giovanni De Lellis, head of the international research team, in a statement released by INFN.

The OPERA experiment is a fast-moving, long-distance game of catch, with CERN laboratory at the border of France and Switzerland pitching a concentrated beam of neutrinos toward the 1,250-ton OPERA detector.

The beam of neutrinos from CERN is made up of muon neutrinos, the same kind of neutrinos created in the Sun. In the 1960s, physicists on multiple experiments found their counts of muon-flavored solar neutrinos were all coming in too low. It turned out that the solar neutrinos were there; they had just changed identities.

The OPERA experiment is the first neutrino experiment to examine a manmade beam of muon neutrinos in search of this type of oscillation. It will continue to take data for the next two years.

The OPERA international experiment at the INFN Gran Sasso Laboratory (Italy) has observed a third neutrino tau candidate from “flavour” oscillation. The “muon-type” neutrino produced at CERN in Geneva arrived at the Gran Sasso laboratory as a “tau” neutrino. An extremely rare event observed only twice before, in 2010 and in 2012. The OPERA international experiment (involving 140 physicists from 28 research institutes in 11 countries) was set up for the specific purpose of discovering this exceptionally rare event. Its observation confirms something scientists have been studying for more than 40 years: the fact that far fewer neutrinos seem to arrive from the Sun and the Earth atmosphere than expected. These “missing neutrinos” are indeed those that have oscillated into a different flavour.
The OPERA experiment was set up in 2001 for this specific purpose. A beam of neutrinos produced at CERN in Geneva travels towards the underground laboratory at the INFN Gran Sasso facility. Thanks to their extremely rare interactions with matter, after travelling through the earth for some 730 km the neutrinos arrive unperturbed at the giant OPERA detector (more than 4,000 tonnes, a volume of approx. 2,000 m3 and nine million photographic plates) where the minute quantity of particles that are caught are observed. In nature there are three kinds of neutrinos, termed “flavours”: electron, muon and tau. OPERA looks for the tau neutrinos knowing that all those leaving CERN are muon neutrinos. When neutrinos of another “flavour” are detected this is proof that oscillation occurs during the 730 km journey. After the first neutrinos arrived at the Gran Sasso laboratory in 2006, the experiment gathered data for five consecutive years, from 2008 to 2012. The first tau neutrino was observed in 2010, the second in 2012.
According to the head of the international research team, Giovanni De Lellis, from the Federico II University and INFN in Naples, the arrival of the third tau neutrino candidate “is an important confirmation of the two previous observations. This event has certain characteristics that make it entirely different from other processes. Statistically speaking too, the observation of three tau neutrino candidates provides the evidence of oscillations in the muon to tau neutrino channel in appearance mode. The data analysis will be pursued for two more years searching for other tau neutrinos that could definitely prove this very rare phenomenon.”

For the new measurement, CERN operators spaced particle bunches in the neutrino beam farther apart by as much as 524 nanoseconds and sent them to Italy in short, three nanosecond pulses. This allowed OPERA physicists to trace neutrinos measured at the final destination back to the exact pulse from which they came. Last time the neutrino bunches were so close together that physicists had to rely on statistics to determine which one corresponded to each observed neutrino. The new arrangement got rid of at least this source of potential fuzziness.

Although this narrower, sparser beam led to more accurate definitions of the particles’ velocities, it also meant that physicists had fewer events to observe overall. In fact, OPERA measured only 20 events this way – one reason the collaboration will need more measurements before concluding anything with certainty.

“Scientists modified the beam of neutrinos traveling through the Earth from CERN to INFN. Image: Jean-Luc Caron

“Since 21 October, CERN has been sending a new type of neutrino beam to Gran Sasso. The new configuration is intended to allow OPERA to define the departure time of the neutrinos more accurately and thus check the previous results obtained using the nominal beam configuration.

The CERN Neutrino to Gran Sasso (CNGS) beam no longer operates using the standard beam time structure. Instead, a new type of proton pulse is being produced by CERN’s accelerators and sent to the graphite target to generate neutrinos. ‘We are now producing extremely short beam pulses,’ explains Edda Gschwendtner, the physicist in charge of the CNGS secondary beam. ‘During a CNGS cycle we now have a LHC type bunched beam with four bunches, each about 2 ns long. Each bunch contains more than 2.5 x 1011 protons; bunches are spaced by 500 ns. In total, this makes about 1012 protons on target for each extraction from the SPS.’

The CNGS beam was originally designed to maximize the number of muon neutrinos produced and sent to Gran Sasso. This was done to increase the probability of observing the “oscillations” as muon neutrinos turn into tau neutrinos. However, after the recent measurement of the neutrino’s speed, it has become important for experiments downstream to receive shorter beam pulses to allow for a more precise definition of the departure time. ‘The nominal CNGS beam cycle has two pulses each, 10.5 μs long and has a total of about 4×1013 protons on target per cycle. In the present configuration, we have a factor of 40 fewer protons per cycle, but in 4 much shorter pulses. Thanks to a huge effort from many experts in the accelerator sector and to experience acquired with the LHC beam, it was possible to make this special proton beam available and to send it to the CNGS target,’ says Edda.”